The present invention relates to an integrated semiconductor light sensor device and to a corresponding manufacturing process.
More specifically, the invention relates to process for manufacturing a matrix of integrated colour sensitive photosensors for colour images, for instance for colour images or a colour television camera.
As is well known, optical light sensors based on semiconductor properties are widely used for several video image applications.
Various techniques may be used for optical to electrical conversion. One of the most effective is based on electron-hole generation due to the light absorption of a semiconductor reverse biased photodiode.
Since the final effect of the electron-hole generation doesn""t represent the wavelength of the absorbed light in the optical range, this physical mechanism cannot distinguish different colours.
To implement colour sensitivity a series of coloured filters is generally provided between the light source and the photosensitive device.
This is usually implemented by a deposition of an organic coloured resin over the finished semiconductor photosensitive device. This resin stops by absorption all the unwanted colours of the incident light and transmits to the light sensor just the light wavelengths to be selected. In this manner the electric signal generated in the semiconductor device is correlated to the selected colour only.
The photodiodes are integrated on silicon to form a bidimensional matrix. From the top view each diode looks squared with sides of about 5 xcexc. Each diode is electrically insulated from the other adjacent diodes by an isolation region, for instance field oxide.
To clearly detect colour images, the semiconductor matrix includes at least three different kind of staggered diodes, which are sensitive to blue, green and red light respectively.
The main drawback of the photodiodes covered by the organic resin is due to the fact that the manufacturing process step for the resin deposition is a further process step and that the filter absorbs a portion of the incident light reducing the diode sensitivity. Moreover, long exposure to intense light and high temperature may reduce the ability of the organic resins to stop the unwanted colours.
A better prior art solution for providing colour selectivity is disclosed in the European patent No. 0152353 which relates to method and device for obtaining a colour selecting effect based on the wave properties of the light. This method may be implemented during the semiconductor device manufacturing process instead than on the finished sensor.
Very briefly, this method provides the deposition of a stack of inorganic layers over the light sensor device. These layers have suitable thicknesses and indexes of refraction.
By a suitable definition of the stack structure it is possible to obtain the desired spectral transmission of the incident light toward the semiconductor sensor device.
Modulating the interference of the light waves reflected at all the layers interfaces of the stack it is possible to maximize or minimize the light intensity transmitted or reflected by the whole stack in pre-determined wavelength ranges, that is colours.
The method disclosed in the above cited European patent is based on the construction of an optical resonant semiconductor stack formed by the following layers: Monosiliconxe2x80x94Oxidexe2x80x94Polysilicon with the oxide thickness equal to L/2, where L is a given wavelength of the incident light.
The incident light is reflected by the interface monosilicon/oxide and interferes with the light reflected by the other interface oxide/polysilicon. Significant reflection of these interfaces takes place due to the high step of refraction index: N_oxide=1.45/N_silicon=4.
Just the waves having wavelength L, which is the double of the oxide thickness d, exhibit a constructive interference. For wavelengths different from L the phase shift L/2 gradually changes causing reflection to increase and transmission to decrease. The result is a transmission curve of the type xe2x80x9cpass-band filterxe2x80x9d centered at the wavelength L.
For instance, interferential L/2 resonators selecting transmission for Blue, Green and Red light must have the following thicknesses of silicon oxide (which is the L/2 layer) deposited directly on the active areas of a monosilicon layer hosting the photosensitive diodes:
Above the oxide layer a thin polysilicon layer (200 xc3x85) is deposited. Then a further layer of silicon nitride (500 xc3x85) is provided over the polysilicon.
Above the nitride layer a standard isolation and passivation oxide layers may be deposited according to the usual CMOS process.
The nitride layer is used to eliminate an additional unwanted interference of reflected light that would occur between the polysilicon and isolation oxide interface. The intermediate nitride refraction index (N_nitride=2) makes negligible the fraction of light intensity reflected at its interfaces.
Now, according to what above, in a standard CMOS process flow the deposition of the three stacks (L/2 Oxide-Polysilicon-Nitride) is performed after the source/drain implantation of the photodiode active area, just before the poly to metal dielectric deposition.
The deposition of the first oxide layer for the interferential filter requires specific process steps since various oxide layers having different thickness must be defined to assure a filtering effect according to a specific light wavelength. To form the oxide layers selecting the three wanted colours, at least a first, a second and a third oxide deposition are required.
The aim of the present invention is that of reducing the complexity of the process steps required to obtain the interferential filter.
A further object of the present invention is that of obtaining a third oxide layer having a thickness multiple of the L/2 value; so a layer having a thickness of L may also be obtained.
A further aim of the invention is that of rendering simpler the final etching step allowing the use of a single etching apparatus just for the polysilicon layer etching step.
The solution idea behind the present invention is that of using a protecting dielectric, for instance a premetal dielectric, as the third layer of the interferential filter.
According to this solution idea, the invention relates to process for manufacturing a light sensor device in a standard CMOS process, including at least the following phases:
implanting active areas on a semiconductor substrate to obtain at least a first, a second and a third integrated region of corresponding photosensors;
forming a stack of layers having different thickness and refractive index over said photosensors to provide an interferential filter for said photosensors;
wherein:
said stack of layers having different thickness and refractive index is obtained by a deposition of a first oxide stack including at least a first, a second and a third oxide layer over at least one photosensor;
said third oxide layer being obtained by a deposition step of an protecting premetal dielectric layer.
The features and advantages of the invention will become apparent from the following description of an embodiment thereof, given by way of non-limiting example with reference to the accompanying drawings.